Generally, the operation of communication systems is based on a hierarchy of protocol implementations. An example is the implementation of a link layer control protocol, e.g., the RLC protocol, being operated on top of a medium access control layer protocol, e.g., the MAC protocol, and providing services to upper layer protocols.
On the link layer control protocol level, automatic repeat request ARQ protocols are used to provide a reliable data transfer in the communication network. Here, when a transmission of one or more data units failed, the receiver may request a retransmission of these data unit(s). Examples of such automatic repeat request ARQ protocols are the link layer control protocols for GPRS, WCDMA, and long term evolution LTE.
Typically, automatic repeat request ARQ protocols provide an in-order delivery of data units carrying a sequence number to upper layer protocols. I.e., upon retransmission of a missing data unit already received those data units with a higher sequence number in comparison to the missing sequence number are not delivered to the higher layer protocol until successful receipt of the missing data unit.
On the level of the medium access control layer protocol being operated under the link layer control protocol, e.g., for HSDPA, there is implemented a multi-process stop-and-wait hybrid automatic repeat request, e.g., according to the HARQ protocol, to perform retransmissions of transport data units that were not correctly received. However, the use of such hybrid automatic repeat request protocols often leads to situations were the medium access control layer receiver receives transport data units out of order with respect to the order of the transport data units at the transmitter side.
Further, the detection of missing data units on the link layer control protocol level, e.g., for the RLC ARQ protocol, relies on an identification of gaps in the sequence number space. If such a gap is identified, the link layer control protocol will trigger a status message to report that one or more data unit(s) is/are missing.
In order to avoid unnecessary issuance of such status reports, the medium access control protocol will provide data units to the link layer control protocol level in order. This minimizes the triggering of unnecessary status reports and consequently avoids a high number of unnecessary retransmissions of data units and related waste of transmission resources.
The re-ordering mechanism on the medium access control protocol level requires a timing mechanism, e.g., the operation of a timer, indicating expiry when further hybrid automatic repeat request retransmissions of a missing transport data unit cannot be expected any more.
Upon expiry, those transport data units that have been received successfully are forwarded to the link layer control protocol level, potential gaps are detected, and finally missing data units are requested to be retransmitted by the link layer control protocol.
However, the existing solution outlined above requires the use of multiple sequence numbers for each protocol layer implementation.
On the medium access control protocol level there are used transport sequence numbers with respect to transport data units. They are used to put transport data units or blocks back into order if hybrid automatic repeat request retransmissions led to a reordering thereof.
Further, on the link layer control protocol level there are used sequence numbers for the operation of the automatic repeat request ARQ protocol to achieve in order delivery of received data units to upper layer protocol levels, which implies significant overhead.